CGM2 I: The Extent of the Circumgalactic Medium Traced by Neutral Hydrogen

We present initial results from the Cosmic Origins Spectrograph (COS) and Gemini Mapping the Circumgalactic Medium (CGMCGM ≡ CGM²) survey. The CGM² survey consists of 1689 galaxies, all with high-quality Gemini-GMOS spectra, within 1 Mpc of 22 z ≲ 1 quasars, all with a signal-to-noise ratio of ∼10 Hubble Space Telescope/COS G130M+G160M spectra. For 572 of these galaxies with stellar masses 10⁷ M_⊙ < M_⋆ < 10¹¹ M_⊙ and z ≲ 0.5, we show that the H I covering fraction above a threshold of N_HI > 10¹⁴cm^-2 is ≳0.5 within 1.5 virial radii (R_vir ∼ R_200m). We examine the H I kinematics and find that the majority of absorption lies within ±250 km s^-1 of the galaxy systemic velocity. We examine H I covering fractions over a range of impact parameters to infer a characteristic size of the CGM, ${R}_{\mathrm{CGM}}^{14}$ , as a function of galaxy mass. ${R}_{\mathrm{CGM}}^{14}$ is the impact parameter at which the probability of observing an absorber with N_HI >10¹⁴ cm^-2 is >50%. In this framework, the radial extent of the CGM of M_⋆ > 10^9.9 M_⊙ galaxies is ${R}_{\mathrm{CGM}}^{14}={346}_{-53}^{+57}$ kpc or ${R}_{\mathrm{CGM}}^{14}\simeq 1.2{R}_{\mathrm{vir}}$ . Intermediate-mass galaxies with 10^9.2 < M_⋆/M_⊙ < 10^9.9 have an extent of ${R}_{\mathrm{CGM}}^{14}={353}_{-50}^{+64}$ kpc or ${R}_{\mathrm{CGM}}^{14}\simeq 2.4{R}_{\mathrm{vir}}$ . Low-mass galaxies, M_⋆ < 10^9.2 M_⊙, show a smaller physical scale of ${R}_{\mathrm{CGM}}^{14}={177}_{-65}^{+70}$ kpc and extend to ${R}_{\mathrm{CGM}}^{14}\simeq 1.6{R}_{\mathrm{vir}}$ . Our analysis suggests that using R_vir as a proxy for the characteristic radius of the CGM likely underestimates its extent.